1. Field of the Invention
The present invention relates to an optical displacement-measuring instrument of a three-grating type.
2. Description of the Related Art
A displacement-measuring instrument such as an encoder has been employed in the art to precisely measure a linear displacement and an angular displacement. Such displacement-measuring instruments are classified into optical, inductive and capacitive types. Among those, an encoder of the optical type comprises a scale provided with an optical grating, and a sensor head movable above the scale, for example. The sensor head includes a light transmissive substrate arranged opposite to the scale and provided with an optical grating; a light source having a light emitting diode; and a photoreceptor having photodiodes.
When the sensor head is moved while the light emitting diode is turned on, the optical grating provided on the scale and the optical grating provided on the light transmissive substrate create a bright and dark pattern of light. This bright and dark pattern is detected at the photodiodes and converted into an electric signal. The amount of displacement can be calculated based on the signal.
The above-described encoder is of the so-called two-grating type that employs the optical grating provided on the scale and the optical grating provided on the light transmissive substrate. In the two-grating type, the smaller the gap between the optical grating on the scale and the optical grating on the light transmissive substrate (the gap between the optical gratings), the smaller the influences such as diffusion of light, and thus an improved resolution can be given to the encoder. A smaller gap between the optical gratings (for example, 10-50 μm), however, makes alignment of the scale with the light transmissive substrate difficult (worsens the alignment characteristic) and requires time and effort in assembly of the encoder. In addition, there is a natural limit of reduction in the gap between the optical gratings.
On the other hand, the so-called three-grating type disclosed in JP-A 2003-279383, FIG. 9, includes a further optical grating (first optical grating) arranged between the light source and the scale. The light from the light source is diffracted at the first optical grating and the diffracted light is diffracted at an optical grating (second optical grating) provided on the scale to create interference fringes, which are received at the photoreceptor through a third optical grating. The interference fringes has an intensity distribution, which is known to have no dependence on the gaps between the optical gratings (the gap between the first and second optical gratings and the gap between the third and second optical gratings). Therefore, the three-grating type is possible to have a larger gap between the optical gratings compared to the two-grating type. Accordingly, the three-grating type is easier to achieve the above alignment (has a better alignment characteristic) compared to the two-grating type.
JP-A 5-87594 discloses an optical encoder in FIG. 1, which includes a mask arranged between a light emitting source and a code plate having slits arranged at a certain interval. An opening is formed through the mask. The opening is operable as an aperture and is not a diffraction grating. Therefore, this encoder fails to correspond to such the three-grating type as described in this specification.